This invention relates to arrays of axial-mode helices in which the interstices between the helices of an array for one frequency are occupied by the helices of an array for a second frequency.
High gain antennas are widely useful for communication purposes and for radar or other sensing use. In general, high antenna gain is associated with high directivity, which in turn arises from a large radiating aperture. A common method for achieving a large radiating aperture is by the use of parabolic reflectors fed by a feed arrangement located at the focus of the parabolic reflector. Parabolic reflector type antennas are very effective, but for certain purposes may present to much of a wind load, and for scanning use they may have too much inertia. Also, parabolic antennas suffer from a problem of aperture blockage due to the structure required to support the feed, which may adversely affect the illumination of the aperture and thereby perturb the far-field radiation pattern.
Those skilled in the antenna art know that antennas are reciprocal transducers, which exhibit similar properties in both transmission and reception modes. For example, the antenna patterns for both transmission and reception are identical, and exhibit the same gain. For convenience of explanation, the explanations are often couched in terms of either transmission or reception of signals, with the other operation being understood. Thus, the term "aperture illumination" may pertain to either a transmission or reception mode of operation.
Modern systems find increasing use of antenna arrays for high gain use. The antenna array includes an array of ordinarily identical antennas or elements, each of which individually often has lower gain than the gain of the array. The antennas are arrayed together and fed with an amplitude and phase distribution which establishes the far-field radiation pattern. Since the phase and power applied to each element of the array can be individually controlled, the direction of the beam can be controlled by control of amplitude and phase. A salient advantage of the array antenna is the ability to scan the beam or beams electronically, without moving the mass of a reflector.
In general, the far-field radiation pattern of an array antenna is the product of the radiation pattern of one of the identical elements, multiplied by the radiation pattern of a corresponding array of isotropic sources. The design of array antennas can be difficult, because an array of isotropic sources in which the array spacing is one wavelength (.lambda.) or greater includes a plurality of grating lobes with a magnitude equal to the main lobe. Ordinarily, such grating lobes are not desired, because when multiplied by the radiation pattern of the individual antenna element, they form sidelobes which allow jamming in a radar context, or perturb the sensing by simultaneously sensing in different directions. Also, such grating lobes represent directivity in unwanted directions which may reduce the gain in the desired direction. If the antennas are placed closer together in order to eliminate the possibility of grating lobes, mutual coupling effects between the antenna elements perturb the radiation patterns of the individual elements in ways which are difficult to predict. Thus, the design of array antennas is only partially amenable to analytic methods.
When array antennas are intended for use at plural frequencies, and grating lobes are not desired, it is ordinarily necessary to space the antenna elements closer than .lambda. at the highest desired frequency of operation, which results in a spacing which is even closer than that which would be required for a separation of .lambda. at a lower frequency. Consequently, when operation at plural frequencies is desired, mutual coupling effects between the antenna elements may be more severe than when single-frequency operation is contemplated. Thus, operation at plural frequencies is often accomplished by two or more completely separate antenna arrays. The size of two or more separate antenna arrays to achieve high gain may result in an unwieldy structure.
An improved antenna array arrangement is desired.